Formyl peptide receptor (FPR) and formyl peptide receptor like 1 (FPRL1) play important roles in inflammation and immunity. Stimulation of FPR and FPRL1 initiates a cascade of signaling events, leading to activation of various phagocyte responses, including chemotaxis, superoxide generation, and exocytosis. Formyl peptide receptor (FPR) and formyl peptide receptor like 1 (FPRL1) are chemoattractant receptors expressed in phagocytic cells, such as neutrophils and monocytes, and play an important role in host defense against pathogen infection. The receptors are known to couple to pertussis toxin (PTX)-sensitive Gi proteins (Le et al., 2000(Le et al., , 2001a. Activation of FPR and FPRL1 induces the dissociation of G␥ subunits from G␣i subunit, which then mediate the activation of phospholipase C (PLC) and phosphoinositide 3-kinase (PI3K) (Le et al., 2000(Le et al., , 2001a. The activation of these effector molecules induces complicated downstream signaling events, which leads to diverse cellular responses, including chemotactic migration, degranulation, and superoxide generation.Most chemoattractants activate multiple signaling pathways that evoke complex immune responses. Many of the induced immune responses are essential for the proper functioning of host cells and for eliminating invading pathogens. However, some immune responses are not beneficial under certain conditions. For the development of therapeutic agents, it is important to enhance the beneficial effects and reduce the adverse effects. To attain this goal, many research groups have attempted to develop selective immune response activator and selective receptor antagonists (Zagorski and Wahl, 1997;White et al., 1998). A larger number of FPR and FPRL1 agonists have been identified either from endogenous sources or by artificial synthesis (Le et al., 2000(Le et al., , 2001a. They include bacterial-derived peptides (fMLF), HIV-envelope protein fragments (T20, T21, F, and V3), and host-derived agonists (Annexin I and A 42 ) (Prossnitz and Ye, 1997;Su et al., 1999;Walther et al., 2000;Le et al., 2001b ABBREVIATIONS: FPR, formyl peptide receptor; FPRL1, formyl peptide receptor like 1; PTX, pertussis toxin; PLC, phospholipase C; PI3K, phosphoinositide 3-kinase; fMLF, formyl-methionyl-leucyl-phenylalanine; WKYMVm, Trp-Lys-Tyr-Met-Val-D-Met-NH 2 ; AM, acetoxymethyl ester; BAPTA, 1,2-bis(2-aminophenoxy)ethane-N,N,NЈ,NЈ-tetraacetic acid; PD98059, 2Ј-amino-3Ј-methoxyflavone; LY294002, 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one; ERK, extracellular signal-regulated kinase; BSA, bovine serum albumin; RPMI, Roswell Park Memorial Institute; MEK, mitogen-activated protein kinase kinase.